This invention relates in general to complementary pair push-pull single ended and bridge power amplifier configurations. Specifically, this invention provides an active feedback biasing circuit for controlling the DC bias level of the various amplifier elements used in the power amplifier.
With the advent of MOSFET power transistors, it became possible to provide, in push-pull and bridge power amplifier configurations, both high-voltage and high-speed operation not previously available with bi-polar power transistors. High-speed, high-voltage operation is highly advantageous in the practical embodiments of linear high fidelity power amplifiers operating in a frequency range up to 500 kHz. Such operation became possible because minority carrier storage does not occur in MOSFETS.
Using a complementary MOSFET power transistor pair such as the Hitachi 2SK135/2SJ50, it is possible to design push-pull and bridge amplifiers that can be operated from 48 volt and 125 volt battery sources. For the high voltage application (125 volts) a conventional bridge circuit, such as the one shown in FIG. 1, is suitable.
Referring now to FIG. 1, there is shown a conventional complementary pair push-pull bridge amplifier configuration. The bridge amplifier includes four MOSFET transistors Q1, Q2, Q3, and Q4 operating as the individual amplifier elements. Transistors Q1 and Q2 form a complementary pair with Q1 being a P-channel enhancement MOSFET and Q2 being an N-channel enhancement MOSFET, Q1 and Q2 forming a complementary pair. Similarly, Q3 is a P-channel enhancement MOSFET and Q4 is an N-channel enhancement MOSFET, Q3 and Q4 forming a complementary pair.
A resistive voltage divider including resistors R1, R2, R3 and R4 provide a gate-source bias voltage V.sub.gs for each of MOSFET transistors Q3 and Q4. In a typical application, R1 is 1K ohm, R2 is 1K ohm, R3 is 60K ohms and R4 is 60K ohms. Thus, ##EQU1## to provide a gate source bias voltage of approximately one (1) volt. Since V.sub.gs is small compared to the drain source voltage V.sub.ds, the feedback ratio H where ##EQU2## is also small. The open loop voltage gain G is of the order of magnitude of 10. The closed loop voltage gain is therefore governed by the following equation: ##EQU3##
With such a high closed loop voltage gain, there is an insufficiently small feedback factor that can be achieved with this circuit. As a result, the center tap voltage V.sub.M of output transformer T1 can deviate substantially from the intended V.sub.b /2 due to any mismatch in the P/N channel transistor characteristics. Gate resistors R.sub.g are intended to increase input impedance. The biasing arrangement for Q1 and Q2 is similar to that provided for Q3 and Q4 previously described and therefore will not be further described here.
The bridge amplifier circuit shown in FIG. 1 incorporates a common source configuration in order to take advantage of the current limiting property of a MOSFET power transistor for short circuit protection. If a common drain configuration were used, the input drive voltage V.sub.i and -V.sub.i would be approximately equal to the high output voltage. Any short circuit condition in the load circuit R.sub.L of the secondary winding of transformer T1 would destroy the power transistors unless a fast protection circuit were provided. A common source circuit shown in the Figure survives output short circuit conditions even without such external protection. The self protection feature of the common source configuration for MOSFET amplifiers results from the absence of a "second breakdown" effect.
Using the conventional complementary pair push-pull bridge amplifier configuration shown in FIG. 1, that utilizes only a resistive voltage divider for biasing, a biasing problem occurs. Since the P and N channel transistor characteristics are not precisely matched, the voltage V.sub.m at the center tap of output transformer T1 is not equal to one-half of the supply voltage, even if adjusted for low and high output power levels. Under actual operating conditions V.sub.m can vary drastically. This variation can result in an unequal power dissipation distribution in the four power transistor amplifier elements. One known approach to solving the biasing problem is to apply passive local feedback to the bridge amplifier, i.e., the use of series-series feedback with source resistors. It has been found however, that series-series feedback with source resistors is either ineffective due to the much smaller transconductance of MOSFETS compared with bi-polar transistors or for large source resistors, the power loss in the resistors is too large. Local parallel-parallel feedback with drain-gate resistors has also proven ineffective since the feedback factor cannot be made sufficiently large if a high voltage transistor has a low threshold or turn-on gate source voltage. Thus, no satisfactory solution to the biasing problem has been found.